Test instruments are commonly used to evaluate operating characteristics of electrical equipment such as integrated circuits, communication systems, and so on. For example, vector network analyzers (VNAs) are commonly used to evaluate the response characteristics of radio-frequency (RF) or microwave frequency circuits or systems.
To ensure accurate performance of a test instrument, a calibration procedure is typically performed on the instrument before it is used in practice. For example, a VNA may be calibrated by measuring the response characteristics of its receivers and then adjusting subsequent receiver measurements for improved accuracy according to the measured characteristics. Such a calibration procedure may be performed, for instance, when manufacturing, initializing, or first deploying the test instrument.
In a conventional calibration procedure, the test instrument is connected to a device under test (DUT) having a known response. This DUT may be referred to as a calibration standard, a reference device, or a known device. The DUT's known response may be, for instance, a known amplitude response, phase response, or group delay response. Next, the test instrument measures the response of the calibration standard and compares the measured response to the known response. Finally, the test instrument is adjusted or calibrated according to any difference between the known response and the measured response. More specifically, the test instrument's measurement parameters are adjusted no that the measured response will correspond more precisely to the known response.
As an example of the above calibration procedure, suppose the test instrument is a VNA and the calibration procedure is used to calibrate the amplitude response of the VNA's receivers. Further suppose that the known response is represented as “K” and the measured response is represented as “M=K+α”, where α represents the difference between the known and measured response. Under these circumstances, the VNA receivers can be calibrated by adjusting their amplitude measurements by a correction factor corresponding to the value of Δ so that future measurements will be consistent with the known response.
In general, the above calibration procedure requires the ability to reliably measure the response characteristic of interest. For example, when calibrating amplitude response, the calibration procedure must be able to reliably measure signal amplitude at an input and an output of the calibration standard in order to measure a change in amplitude between the input and output. This type of amplitude measurement can typically be carried out in a straightforward manner using a power meter to detect respective signal amplitudes at the input and output. In other types of calibration, however, more sophisticated techniques may be required to perform the relevant measurements. For instance, when calibrating the phase response of a test instrument using a frequency translation device (FTD) (e.g., a mixer) as the calibration standard, it may be difficult to determine phase shifts between the input and output of the calibration standard due to the change of frequency between the input and output. This difficulty stems from the fact that there is currently no available general purpose “phase meter” (i.e., analogous to a “power meter”) capable of detecting absolute phase levels at the input and output of an FTD.
To address the above difficulty, a new technique has been developed for calibrating the phase response of a test instrument using a harmonic comb generator as the calibration standard. In other words, the new technique can perform phase calibration as long as the calibration standard is a harmonic comb generator having a known phase response. In this context, such a harmonic comb generator can be referred to as a “phase reference”, and the calibration technique can be referred to as a “phase reference” calibration technique. Examples of the phase reference calibration technique are described in further detail in commonly-owned U.S. Patent Application Publication No. 2012/0295548 filed May 18, 2011 (the '548 Publication), the subject matter of which is hereby incorporated by reference.
Unfortunately, the phase reference technique has at least two significant shortcomings. First, in most practical contexts, the phase reference has a limited range of operation. For instance, it may operate over a range of 55 MHz to 67 GHz. Accordingly, the phase reference may not be used to calibrate phase response of the receivers of a VNA or other test instrument at frequencies below 55 MHz or above 67 GHz. Second, the phase reference is generally limited to discrete frequencies, referred to as cardinal frequencies. For instance, a typical phase reference may comprise a harmonic comb generator that generates signals at multiples of 10 MHz, e.g., 60 MHz, 70 MHz, etc. The phase reference cannot be used to directly calibrate phase response between those frequencies, nor does a typical calibration lend itself to interpolation between those frequencies.
In view of the foregoing, there is a general need for new approaches to calibration of test instruments that overcome the shortcomings of conventional approaches.